Enzymes that can form or break carbon-carbon bonds are sought-after biocatalysts. Tuning them to work on all manner of substrates can be challenging. Now, however, chemists at Texas A&M University have shown that the most abundant protein in whey, β-lactoglobulin, can catalyze retroaldol cleavage of α,β-unsaturated aldehydes (Org. Biomol. Chem. 2018, DOI: 10.1039/c8ob00139a). This milk protein’s natural function isn’t known, but Coran M. H. Watanabe’s team previously showed that it can promote condensation of α,β-unsaturated aldehydes. Watanabe’s group surmised that β-lactoglobulin would have retroaldolase activity as well, because its active-site lysine resembles that of another group’s de novo-designed retroaldolase. The team’s experiments showed this to be the case. The enzyme works best on substrates with hydrophobic phenyl and naphthyl side chains, thus complementing the selectivity of similar enzymes that work best on hydrophilic substrates. The team increased the enzyme’s yield by blocking a competing reaction. Whey waste abounds in the dairy industry, so although the enzyme’s turnover numbers are on the low side, easy availability makes it a biocatalyst worth exploring, the team writes.